The present invention relates generally to code division multiple access (CDMA) cellular communications systems. More specifically, but without limitation thereto, the present invention relates to assigning Walsh codes to service a maximum number of users in a code division multiple access service area having varying data rates.
In third generation code division multiple access cellular communications systems, one of the mechanisms used to provide variable-rate data transmission is to use different length Walsh-Hadamard codes (hereafter referred to as Walsh codes) for modulation on forward link communications signals. Walsh codes of the same length are orthogonal to one another, and may therefore be used in the same frequency band by different users without interfering with one another. Users having slower data rates are assigned longer Walsh codes, while users having faster data rates are assigned shorter Walsh codes.
A problem arises in assigning combinations of different length Walsh codes, because the shorter Walsh codes “cover” certain longer Walsh codes, i.e., longer Walsh codes generated from shorter Walsh codes are not orthogonal to the shorter Walsh codes. Mutual interference could result if shorter Walsh codes were assigned at the same time as longer Walsh codes that include the shorter Walsh codes. To service the maximum number of users having different data rates, each Walsh code assigned from the Walsh code space preferably excludes from use the minimum number of shorter Walsh codes. Current methods for searching for the next available Walsh code are time consuming and do not necessarily exclude the minimum number of shorter Walsh codes.
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